Astigmatism is a defect of the eye wich has the consequence of causing a point source situated at infinity to give rise to an image comprising two small perpendicular segments of light which are spaced apart from each other along the optical axis by a distance which is a function of the degree of astigmatism. These two segments correspond to the focal lengths of the astigmatic beam and they may be situated on the same side of the retina, either in front of it (myopic astigmatism) or behind it (hyperopic astigmatism), or else they may be situated on either side of the retina (combined astigmatism). There are two possible sources for such defects: either the cornea or else the lens of the eye; with the cornea and the lens having surfaces which are not surfaces of revolution, and which have two main radii.
Astigmatism of the eye is corrected by compensating for said aberration so that a point object gives rise to a spherical wave centered on an image point on the retina.
In the past, this correction has been obtained using sphero-cylindrical lenses which correspond functionally to associating a spherical lens with a cylindrical lens.
Such lenses have already provided good service. However, accurately defining sphero-cylindrical surfaces suitable for obtaining optimum correction is often difficult and complex.
For several years, the person skilled in the art has been considering developing presbyopia-compensating lenses comprising diffractive components (see for example European patent EP-A-0 064 812, U.S. Pat. No. 4,637,697, or French patent applications FR-88 06 699 filed May 19, 1988 and FR-88 14 634, filed Nov. 9, 1988, in the name of the present Applicant).
To this end, diffractive lenses proposed so far comprise a series of diffractive structures disposed concentrically around the optical axis of the lens.
The present invention now seeks to propose novel means for correcting astigmatism of the eye by utilizing diffractive components.